Linear mode avalanche photodiodes without excess noise

ABSTRACT

A linear mode avalanche photodiode senses light and outputs electrical current by being configured to, generate a gain equal to or greater than 1000 times amplification while generating an excess noise factor of less than 3 times a thermal noise present at or above a non-cryogenic temperature due to the gain from the amplification. The linear mode avalanche photodiode detects one or more photons in the light by using a superlattice structure that is matched to suppress impact ionization for a first carrier in the linear mode avalanche photodiode while at least one of 1) increasing impact ionization, 2) substantially maintaining impact ionization, and 3) suppressing impact ionization to a lesser degree for a second carrier. The first carrier having its impact ionization suppressed is either i) an electron or ii) a hole; and then, the second carrier is the electron or the hole.

TECHNICAL FIELD

Embodiments of this disclosure relate generally to photodiodes.

BACKGROUND

The function of a photodetector is to sense incident light and to outputan electrical current, the photocurrent, proportional to the incidentlight flux. Ideally, the photodetector produces one output electron orhole for every incident photon and every output electron or hole is theresult of an incident photon. For many applications where the incidentflux is low, the output current requires amplification before it can beused by subsequent electronics. In some previous photodetectors, inorder to achieve photon counting, then a gain mechanism for multiplyingthe photocurrent by 10⁴ to 10⁶ is needed to detect a single photon.Besides increasing the amplitude of the current, amplifiers have thedrawback of adding noise to the current.

Practically, the noise current from electronic amplifiers at roomtemperature for many types of photodetectors is well above that requiredfor detecting a stream of single photons (photon counting). Since theamplifier noise is temperature-driven, for these type of photodetectors,then cooling the amplifier to cryogenic temperatures would lower itsnoise but this approach is acceptable only for a limited number ofapplications.

One current multiplication mechanism is impact ionization.

Photodetectors based on this mechanism are known as avalanchephotodiodes (APDs).

SUMMARY

Provided herein can be various methods, apparatuses, and systems forphotodiodes.

In an embodiment, a linear mode avalanche photodiode senses light andoutputs electrical current by being configured to, generate a gain equalto or greater than 1000 times amplification while generating an excessnoise factor of less than 3 times a thermal noise present at or above anon-cryogenic temperature due to the gain from the amplification. Thelinear mode avalanche photodiode detects one or more photons in thelight by using a superlattice structure that is matched to suppressimpact ionization for a first carrier in the linear mode avalanchephotodiode while at least one of 1) increasing impact ionization, 2)substantially maintaining impact ionization, and 3) suppressing impactionization to a lesser degree for a second carrier. The first carrierhaving its impact ionization suppressed is either i) an electron or ii)a hole; and then, the second carrier is the electron or the hole.

DRAWINGS

FIG. 1 illustrates an embodiment of a block diagram of a linear mode APDwith a matched superlattice structure in which the excess noise issubstantially eliminated via having a superlattice in the valence bandwhen the carrier is a hole.

FIG. 2a illustrates an embodiment of a graph of a hole in the valenceband of a bulk semiconductor under bias accruing kinetic energy withdistance.

FIG. 2b illustrates an embodiment of a graph of a hole in the valenceband hopping through wells in a biased superlattice and the energylevels for states in a Wannier-Stark ladder.

FIG. 3 illustrates an embodiment of a graph of a theoretical value ofthe excess noise factor F as a function of the average gain M and theβ/α ratio for an electron injection.

FIG. 4 illustrates an embodiment of a graph of the band diagram of asemiconductor where energy is plotted vertically and distancehorizontally and an amount of kinetic energy needed to impact ionize toform a new electron-hole pair.

FIG. 5 illustrates an embodiment of a block diagram of a linear mode APDwith a matched superlattice structure in which the excess noise issubstantially eliminated via having a superlattice in the conductionband when the carrier is an electron.

FIG. 6a illustrates an embodiment of a graph of an electron in bulksemiconductor with an applied electric field that is accelerated by thefield and accumulates kinetic energy.

FIG. 6b illustrates an embodiment of a graph of the matched superlatticesuppresses impact ionization by controlling the localized Wannier Starkstates.

FIG. 7 illustrates an embodiment of a graph of a linear mode APD with amatched superlattice design where electron multiplication, but not thehole multiplication, is suppressed.

FIG. 8 illustrates an embodiment of a diagram of a linear mode APD arraywith a matched superlattice structure with in-situ memory and a parallelcolumn readout.

FIG. 9 illustrates an embodiment of a diagram of a Read-out circuitschematic with linear mode APDs with a matched superlattice structure.

While the design is subject to various modifications, equivalents, andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will now be described in detail. Itshould be understood that the design is not limited to the particularembodiments disclosed, but—on the contrary—the intention is to cover allmodifications, equivalents, and alternative forms using the specificembodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details can be setforth, such as examples of specific data signals, named components,number of frames, etc., in order to provide a thorough understanding ofthe present design. It will be apparent, however, to one of ordinaryskill in the art that the present design can be practiced without thesespecific details. In other instances, well known components or methodshave not been described in detail but rather in a block diagram in orderto avoid unnecessarily obscuring the present design. Further, specificnumeric references such as the first photodiode, can be made. However,the specific numeric reference should not be interpreted as a literalsequential order but rather interpreted that the first photodiode isdifferent than a second photodiode. Thus, the specific details set forthcan be merely exemplary. The specific details can be varied from andstill be contemplated to be within the spirit and scope of the presentdesign. The term “coupled” is defined as meaning connected eitherdirectly to the component or indirectly to the component through anothercomponent.

In general methods, apparatuses, and systems are discussed. An array ofa plurality of linear mode avalanche photodiodes with a matchedsuperlattice structure may be used in a system. Each linear modeavalanche photodiode senses light and outputs electrical current bybeing configured to, generate a gain equal to or greater than 1000 timesamplification while generating an excess noise factor of less than 3times a thermal noise present at or above a non-cryogenic temperaturedue to the gain from the amplification. The linear mode avalanchephotodiode detects one or more photons in the light by using asuperlattice structure that is matched to suppress impact ionization fora first carrier in the linear mode avalanche photodiode while at leastone of 1) increasing impact ionization, 2) substantially maintainingimpact ionization, and 3) suppressing impact ionization to a lesserdegree for a second carrier. The first carrier having its impactionization suppressed is either i) an electron or ii) a hole; and then,the second carrier is respectively the electron or the hole. The powersupply is used to power the plurality of linear mode avalanchephotodiodes.

FIGS. 1 and 5 illustrate a linear mode APD 100, 500 with a cap layer, anabsorption layer, a matched superlattice structure in the superlatticemultiplication region, and a collector layer. A voltage controllerapplies a reverse bias to the linear mode APD 100, 500. The linear modeAPD 100, 500 senses light and outputs electrical current by beingconfigured to, generate a potentially exponential gain equal to orgreater than 1000 times amplification while generating an excess noisefactor of less than 3 times a thermal noise present at or above anon-cryogenic temperature due to the gain from the amplification. Thesuperlattice structure is matched to suppress impact ionization forfirst carrier in the linear mode APD 100, 500 while maintaining impactionization for the other carrier. The first carrier having its impactionization suppressed is either i) an electron or ii) a hole; and then,the other carrier maintaining its impact ionization is the electron orthe hole. We suppress the impact ionization of a carrier by creating asuperlattice in the valence band when the carrier is a hole (see FIG. 1)and with a superlattice in the conduction band when the carrier is anelectron (see FIG. 5). A non-cryogenic temperature may be warmer than,for example, minus 50 degrees C.

Thus, the linear mode APD 100, 500 with the matched superlatticestructure is configured in the superlattice multiplication region tosuppress the impact ionization of the first carrier i) by implementing asuperlattice in a valence band when the first carrier is a hole and ii)by implementing a superlattice in a conduction band when the firstcarrier is an electron. The gain from impact ionization can only beinitiated by the other carrier with kinetic energy (KE) greater than thebandgap energy (Eg), through the use of a matched superlattice; as wellas, the first carrier, either the electron or the hole, is preventedfrom accumulating enough kinetic energy under an electric field large toimpact ionize.

A cooperation exists between the superlattice structure and the voltagecontroller for the fixed or variable electric field applied to thesuperlattice structure. A periodic structure of layers of two or morematerials forming the matched superlattice structure is designed to haveat least one quantum state in each of its wells when the superlattice isbiased by a voltage controller to have an electric field that wouldsustain impact ionization in a thick layer of a well semiconductor or abarrier semiconductor alone. The periodic set of materials for thematched superlattice structure is designed also to have a large spatialoverlap of wavefunctions of the quantum states of adjacent wells whenunder an applied bias such that the first carrier traverses thesuperlattice by hopping from a first well to a second well andsubstantially loses an amount of energy supplied by the applied bias toend up being less than a kinetic energy needed to impact ionize to forma new electron-hole pair for each hop from well to well. A range of biasfrom the voltage controller will facilitate these objectives.

The large spatial overlap promotes the rapid decay of a carrier in thelocalized state of a particular well into the lower energy localizedstate of the adjacent state. This process is known as hopping. Theenergy difference is lost to the emission of phonons. The carriertraverses the superlattice by hopping from one well to the next well andlosing the energy supplied by the applied bias to phonons at every step.

The matched superlattice structure in the multiplication region can giveband discontinuity in only one band (gain without noise) via the matchedmaterials that can have 100's of periodic sets of nanometer-thick layersto allow precise shaping of the band energy structure. The materialsforming the matched superlattice structure in the multiplication regionperiodically repeat.

The linear mode APD 100, 500 with the matched superlattice structure ismatched to suppress the impact ionization for only the first carrier inthe superlattice multiplication region by using a periodic structure oflayers of two or more materials grown on a substrate that can be chosenfrom lattice matched pairs of semiconductors to form the superlatticestructure. The lattice matched pairs have their band offsets set to i)at least mostly be in a band corresponding to the suppressed carrier andii) up to entirely be in the band corresponding to the suppressedcarrier where impact ionization is to be suppressed. The lattice matchedpairs of semiconductors forming a period of the superlattice structureconsists of at least one pair of alternating layers of a firstsemiconductor material and a second semiconductor material. Thesuperlattice consists of a repetition of multiple periods where thecorresponding thicknesses of each layer of a period are the same. All ofthe layers of the first semiconductor have substantially a samethickness without doping. All of the layers of the second semiconductorhave substantially a same thickness without doping. However, thethickness of the layers of the first semiconductor can be different thanthe thickness of the layers of the second semiconductor.

Note, the unsuppressed carrier in the other band will not be in asuperlattice at low energy, so its transport will proceed qualitativelyas if were in a bulk semiconductor. The unsuppressed carrier in theother band will impact ionize to provide the gain needed for the linearmode APD. With the impact ionization occurring in the opposite band ofthe first carrier being suppressed, then the excess noise will beeliminated.

The linear mode APD 100, 500 with the matched superlattice can operateover a wide range of light detection including across UV, Visible, andShort-wave IR (˜2500 nm) light waves.

In an embodiment, the linear mode APD 100, 500 can detect single photonswith high quantum efficiency near room temperature. However, previousphotodetectors without a matched superlattice, the noise they generatecan overwhelm the detection signal from the photons. The linear mode APD100, 500 with the matched superlattice can detect a single photonwithout a penalty of having a deadtime after the detection of the singlephoton where the linear mode avalanche photodiode cannot detect anotherphoton until after a set period of time. Thus, no pixel “dead-time” upona detection event (=“real-time” data acquisition.) The linear modeavalanche photodiode with the matched superlattice multipliesphotocurrent above the noise-equivalent input current of the electronicsthat will amplify the signal of a single photon to a measurable voltagewhile adding very little noise in the multiplication process.

The APD 100, 500 with a matched superlattice behaves as a linearamplifier so, in contrast to a Geiger mode APD, the linear mode APD 100,500 with a matched superlattice can resolve the near simultaneousarrival of multiple photons and it does not have a dead time after itdetects a photon. The APD 100, 500 can deliver a room-temperature,InP-based, linear mode APD with a gain of 10⁴ electrons per photon, lowexcess noise, and high quantum efficiency that will achieve the ultimatein detector sensitivity. The linear mode APD 100, 500 with the matchedsuperlattice can simultaneously achieve single-photon sensitivity at orgreater than, for example, room temperature, with no dead time andminimal excess noise.

Next, typically, a general APD has a large internal gain, M, that canpotentially multiply the photocurrent tremendously. However, itsperformance has been disappointing because the noise added by themultiplication process, known as excess noise, overwhelms the amplifiedsignal even at a modest value of M≈_50. The origin of the excess noisethe simultaneous presence of electron-initiated impact ionization andhole-initiated impact ionization. Together, they create a positivefeedback loop that magnifies small fluctuations in the impact ionizationprocess. Two parameters used to characterize an APD are theelectron-initiated impact ionization coefficient α _and thehole-initiated impact-ionization coefficient β. The excess noise factor,F, is≤_2 for all values of M when either α _or β _is zero (unipolargain), but F is proportional to M for large M for bipolar gain (see FIG.3).

FIG. 3 illustrates a graph 300 of a theoretical value of the excessnoise factor F as a function of the average gain M and the β/α ratio foran electron injection. The β/α curve of 0.03 is the current best valuefor an APD at room temperature. The β/α curve shows that for M>50, Fgrows with increasing M. The matched superlattice can have a β/α_<<10⁻⁴.

A linear mode APD with the matched superlattice has a unipolar gain andwill behave like a photomultiplier tube (PMT) but with the tremendoussystem advantages in area occupied by the device and of a ruggednessassociated with the semiconductor device. By providing high gain withlow noise within the detector, the unipolar gain APD can photon count inthe linear mode—its output can be coupled to commercially availableelectronic amplifiers to give a waveform in which the arrival of asingle photon is detectable above the noise and in which thesimultaneous arrival of multiple photons can be distinguished.

Without a room-temperature APD technology that can provide enoughlow-excess-noise gain for linear-mode photon counting, the alternativeis photon counting in the Geiger mode. In this mode, the APD is biasedabove its breakdown voltage to a metastable state in which no currentflows until it is triggered by the arrival of a single photon or by darkcurrent. Once triggered, the APD current rapidly grows to a value thatcan be easily detected. While the Geiger mode APD is capable ofsingle-photon detection, it has two serious drawbacks: (1) it has a deadtime of ˜100 nanoseconds and longer after it is triggered and duringwhich it has no photo response; and (2) it has after pulsing—there-emission of carriers captured by trap states during the avalanchethat gives a higher dark count rate for many microseconds after beingtriggered. The first of these increases the false-negative rate and thesecond the false-positive rate for applications, such as LIDAR, in whichthe arrival of the signal photons is clustered in time.

Next, FIGS. 1 and 5 show a linear mode APD 100, 500 with a matchedsuperlattice structure in which the excess noise is substantiallyeliminated by creating an enormous asymmetry between the electron andthe hole transport. The superlattice structure has a set of materialswith the desirable unipolar impact ionization using constituents that donot have any special impact-ionization properties; and thus, can bepotentially employed for APDs 100, 500 spanning material system thatcollectively are used to detect a wide range of wavelengths. The gainregion is composed of two or more lattice-matched semiconductor alloys.The matched two or more lattice-matched semiconductor alloys have theproperty that their entire band offset is only in either the valenceband or the conduction band. See FIGS. 1 and 5 for band offset is onlyin either the valence band or the conduction band.

FIG. 2a illustrates an embodiment of a graph of a hole in the valenceband of a bulk semiconductor under bias accruing kinetic energy withdistance. FIG. 2b illustrates an embodiment of a graph of a hole in thevalence band hopping through wells in a biased superlattice and theenergy levels for states in a Wannier-Stark ladder. FIGS. 1, 2 a, and 2b show gain from electron multiplication only with the superlattice inthe valence band. Similarly, FIGS. 5, 6 a, 6 b, and 7 shows gain fromhole multiplication only with the superlattice being in the conductionband instead of valence band.

Impact Ionization in APDs

FIG. 4 shows a band graph 400 of a semiconductor where energy is plottedvertically and distance horizontally and an amount of kinetic energyneeded to impact ionize to form a new electron-hole pair. Free electronsreside in the conduction band and free holes in the valence band. Theconduction band and the valence band are separated by the bandgap ofwidth Eg where there are no allowed electron or hole states. With anapplied electric field, the bands are tilted in proportion to themagnitude of the field as shown in FIG. 4. The electron at the top lefttravels to the right as it is accelerated by the applied field. Thevertical distance between the electron's instantaneous position and theconduction band is the instantaneous kinetic energy of the electron.Once the kinetic energy of the electron exceeds the bandgap energy, ithas enough energy to create an electron-hole pair by impact ionization.When the electron does impact ionize, most of the kinetic energy goesinto creating an electron-hole pair and results in two electrons and onehole with very little kinetic energy. Electron-initiated impactionization is characterized by a coefficient α, the average number ofelectron-initiated impact ionization per centimeter (See FIG. 1).

A similar chain of events can happen in the valence band with a holeaccelerating to create an electron-hole pair. In this case, there aretwo holes and one electron after the hole-initiated impact ionization(See FIG. 5). This process is characterized by the hole-initiated impactionization coefficient β.

FIG. 1 shows the graphs of the two bands overlaid on the superlatticemultiplication region. Both electrons and holes are accelerated by theelectric field, so both will initiate the creation of a newelectron-hole pair by impact ionization. The general case will bediscussed in the next section, but the case where β=0 can be easilyunderstood. A single electron impact ionizes, resulting in two electronsand a hole. The two electrons each accelerate and leave four electronsafter impact ionization. The four electrons become eight, etc., etc., sothe number of electrons grow exponentially with distance z as e^(αz).The number of holes also grows exponentially since a hole is createdevery time an electron is created. Since these holes do not impactionize when β=0, they do not increase the number of electrons. So whenβ=0, the semiconductor provides an exponential gain e^(αz) with the onlynoise arising from variations in the distance an electron travels beforeit impact ionizes. Similarly, α=0 gives an exponential gain e^(βz).

Excess Noise in APDs

In some previous techniques, for most semiconductors, the value of α andof β are very similar so the simple case of exponential gain does nothold. Consider the case where a photogenerated electron is injected fromthe absorption layer into a biased semiconductor layer at z=0 andinitiates the first impact ionization at z=z₀.

A second electron and a hole are generated by the impact ionization. Thehole, having the opposite charge as an electron, drifts in the oppositedirection as the electrons and will impact ionize to create a newelectron-hole pair at z=z₁ somewhere between z=0 and z=z₀ if it does notreach the end of the semiconductor layer first. There is some randomvariation in how far the hole drifts before it impact ionizes because ofrandomness in where and when it scatters with a crystal defect or aphonon. These variations can make a large difference near the end of thelayer by determining whether or not the hole impact ionizes just beforereaching the end. The newly generated electron at z=z₁ will drift towardz=z₀, perhaps impact ionizing along the way to create more electron-holepairs. Even without considering the electron-hole pairs that the twoelectrons emerging from the first impact ionization will subsequentlygenerate, it should be clear that the single initial photoelectronimpact ionizing at z=z₀ generates subsequent electrons passing throughz=z₀. However, it is impossible to distinguish if a second electronpassing though z=z₀ is a byproduct of the initial photoelectrongenerated by the sequence of impact ionization as just described or is asecond photoelectron injected into the layer. The signal in the firstsituation is due to the injection of only the initial electron and inthe second situation is due to the injection of two electrons atslightly different times. Since the impact ionization obscures thedifference between these possible inputs, the process adds noise to thesignal. This noise is known as excess noise. Excess noise in APDs hasbeen well studied.

FIG. 3 shows the excess noise factor F as a function of β/α and theaverage APD gain M for electron injection into the gain region. Theβ/α=0 curve gives the lowest F for all M and it is the β=0 case wedescribed above. The result that F has a value of 2 rather than a valueof 1 expected for the ideal case is due to an assumption made in thecalculations that the impact ionization is a Poisson process. As β/αincreases, the excess noise factor remains at F=2 at low M but increasesat high M. Closer inspection of the curves yields the the rule that toachieve a gain M with F=3 requires that α/β≈M. Achieving a gain of 10⁴,enough to detect single photons under some conditions, with littleexcess noise requires a semiconductor with β/α=10⁻⁴. So far, no singlecomponent semiconductor with this large of a difference between α and βis known, so an engineered material with alloys is needed to achievethis difference and to achieve gain being initiated for merely firstcarrier type (an electron or a hole.)

In FIG. 3 as F gets large, then M gets large, unless alpha/beta orbeta/alpha gets very small. The current design is how to make a verysmall alpha/beta or beta/alpha and thus get a small F for a large M.

Another consequence of simultaneously having α≠0 and β≠0 is avalanchebreakdown. Breakdown occurs when the number of electrons or holesbecomes arbitrarily large for a finite voltage and for a multiplicationregion of finite size. As discussed earlier for α=0 or β=0, the numberof electrons and holes grow exponentially with z. This number, althoughit is growing rapidly, remains finite for finite voltage (implying thatα and β remain finite) and for finite size (implying z remains finite).

However, the situation changes when α≠0 and β≠0. Suppose that a singleelectron is injected into the multiplication region at z=0 and initiatesan impact ionization at z=z₁. The hole created at this impact ionizationdrifts in the −z-direction and can impact ionize before arriving at theend of the multiplication region at z=0. When it does, it creates anelectron at z somewhere between 0 and z₁. This is exactly the initialsituation repeated at a later time. At the threshold of breakdown, loopssuch as this become self-sustaining so that a single injected electronproduces an infinite number of subsequent electrons. In this simplifiedpicture where we considered only the first impact ionization initializedby each carrier, the build-up of carriers occurs exponentially withtime. A more realistic picture that considers all impact ionizationswould give a much faster increase with time.

As discussed, the linear mode avalanche photodiode senses photons andoutputs electrical current by being configured to generate a gain equalto or greater than 1000 times (10³) amplification while generating anexcess noise of less than 3 times a thermal noise present (β/α<=0.1) ator above a non-cryogenic temperature due to the gain from theamplification. (See the shaded area of operation in the lower right handportion of FIG. 3) In an embodiment, the minimum useable gain may spanbeing equal to or greater than 200 times amplification while generatingan excess noise of less than three times a thermal noise present.

In an embodiment, the linear mode avalanche photodiode with the matchedsuperlattice structure such that amplification occurs in merely i) theconductance band or ii) the valance band is configured to generate again of equal to or greater than 10,000 times (10⁴) amplification whilegenerating an excess noise of less than three times the thermal noisepresent at or above a room temperature due to the gain from theamplification. (β/α<=0.1) The linear mode avalanche photodiode with thematched superlattice structure with the gain of 10,000 timesamplification is capable of detecting a single photon without a penaltyof having a deadtime after the detection of the single photon where thelinear mode avalanche photodiode cannot detect another photon untilafter a set period of time. (Thus, no pixel “dead-time” upon a detectionevent (=“real-time” data acquisition”). With the deadtime, a waveformcaptured by an APD will have a gap in time after the detection of eachphoton where a subsequent photon cannot be detected.

FIGS. 2a and 2b illustrate a comparison of hole transport in a biasedbulk semiconductor, and in a biased superlattice. Referring to the graph200 in FIG. 2 a, in the bulk semiconductor, the electric fieldaccelerates the hole to the left in an extended state. The kineticenergy of the hole increases as it is accelerated until it has enoughenergy to impact ionize. Referring to the graph 250 in FIG. 2b , theallowed states in a biased superlattice are Wannier-Stark states thatare mainly in one well but extend to a few wells to the left. The groundstate of the well A is also the first excited state of the well B. Ahole in this extended state can relax into the ground state of well B byemitting a phonon or photon. In this way, a hole can be transportedleftward while remaining in the local ground state and never havingenough energy to impact-ionize.

As shown in FIGS. 1, 2 b, 5, 6 b, and 7, a superlattice of, for example,two matched alloys will have wells and barriers in one band but not theother, so carrier transport in the two bands will be very different.Transport in the band with the offset will be by hopping (also known asphonon-assisted tunneling) from a state localized at one well to a statelocalized in an adjacent well. The energy imparted to the carrier by theapplied electric field dissipates through phonons or through midwave orlongwave infrared photons so that the carrier does not accumulate enoughkinetic energy to impact-ionize. Therefore, the superlattice has β=0.

A carrier in the band without the offset does not have the localizedstates needed for hopping. So, as in an ordinary semiconductor underbias, it is in an extended state where it accumulates kinetic energy asit drifts in the electric field, and it impact-ionizes when its kineticenergy exceeds the bandgap energy (see FIGS. 1, 4, and 5). The matchedlinear mode APD structure has a large asymmetry between the ability ofthe electron and hole to impact-ionize and can give an APD 100, 500 withhigh gain (>10⁴) with low excess noise (F<2) at room temperature.

From the above discussion, exponential gain free of excess noise canoccur when α=0 or β=0. To do this, note that impact ionization can onlybe initiated by a carrier with kinetic energy greater than the bandgapenergy. The matched superlattice structure prevents either the electronor the hole from accumulating enough kinetic energy under an electricfield large enough that the opposite carrier can accumulate the kineticenergy to impact ionize. Thus, the gain from impact ionization can onlybe initiated by a carrier with kinetic energy greater than the bandgapenergy through the use of a matched superlattice in which thesuperlattice structure causes either the electron or the hole to beprevented from accumulating enough kinetic energy under an electricfield that is large enough that the opposite carrier can accumulate thekinetic energy needed in order to impact ionize.

The matched linear mode APD 100, 500 eliminates excess noise bysuppressing the impact ionization for first carrier while maintaining itfor the other carrier. Attempts to make a linear mode APD with a largeor small α/β are based on enhancing the impact ionization for firstcarrier while maintaining it for the other. For a given fractionalchange in the impact ionization rate of first carrier, using it forsuppression gives a greater change in α/β. For example, increasing theimpact ionization rate of a carrier by 50% increases α/β by a factor of3/2 while decreasing the impact ionization rate by 50% increases β/α bya factor of 2.

Similar to the hole discussion in FIGS. 2a and 2b , FIG. 6a illustratesan embodiment of a graph 600 of an electron in bulk semiconductor withan applied electric field that is accelerated by the field andaccumulates kinetic energy. FIG. 6b illustrates an embodiment of a graph650 of the matched superlattice suppresses impact ionization bycontrolling the localized Wannier Stark states. A superlattice can be aperiodic structure of layers of two or more materials. Typically, thethickness of a layer of material may be indicated in nanometers. Withthe appropriate band offset and layer thicknesses, the superlatticeunder electrical bias forms Wannier-Stark states that are mostlylocalized to a single well. The energy level of these states isindicated by the dashed lines and their wavefunction by the filledshaded curves. A continuum of extended states in which an electron canaccelerate in the field exists at energies above the barrier energy. Itis imperative that electrons be prevented from occupying the continuum.With proper design of the superlattice, the Wannier-Stark states and thecontinuum are separated by many kT of energy. Thus, electrons in theWannier-Stark states are unlikely to be thermally excited into thecontinuum where they can be accelerated by the electric field.

Note that for an electron in the Wannier-Stark state in well 0, thestate in well 1 is at lower energy and is therefore is more favorable.The electron can make the transition from well 0 to well 1 by emittingphonons or a photon to conserve energy; this process is known ashopping. The probability of hopping is proportional to the overlap ofthe well 0 wavefunction and the well 1 wavefunction. Thinner wells andbarriers and a smaller difference between the barrier and wellconduction band levels lead to larger overlaps but can convert thelocalized states back to the extended states that we are trying toeliminate. An electron hopping from the well 0 state to the well 1 stateaccomplishes two useful functions. An electron hopping from the well 0state to the well 1 state moves the electron by one superlattice period‘a’ in the z-direction and it removes an energy of qEa from theelectron, where q is the electron charge and E is the applied electricfield. This is exactly the kinetic energy that the field imparts to acharge q over a distance ‘a’ in the bulk semiconductor. The electron inthe superlattice has moved a distance a in a field without changing itskinetic energy. Once the electron is in well 1, the electron can hop towell 2 and then to well 3, etc., without acquiring the kinetic energyneeded to impact ionize.

The thickness of the well and barrier layers are determined with thesedesign goals: Wannier-Stark states are formed at the applied electricfield required to give the desired carrier multiplication, the energylevel of the Wannier-Stark states are low enough in the wells thatcarriers are not thermally excited into the continuum, and the states inadjacent wells have a large overlap so hopping can be fast.

The above discussion suppresses electron-initiated impact ionizationwith a superlattice in the conduction band; and accordingly,hole-initiated impact ionization can be suppressed in a similar way witha superlattice in the valence band (See FIGS. 2b and 1). Note though,suppressing impact ionization in one band with a superlattice is halfthe solution to eliminate excess noise in an APD. Picking any twosemiconductors, one for the well and one for the barrier, withoutspecial consideration will very likely lead to a superlattice in boththe valence and the conduction band. Impact ionization will besuppressed in one band by design, but impact ionization in the otherband may also be suppressed or at least reduced. So, it will beadvantageous to choose a pair of semiconductors where the band offsetsare entirely in the band where impact ionization is to be suppressed.

The other band will have no offset and will behave approximately like abulk semiconductor. However, there are two major differences. First, theimpact ionization coefficient of the two materials will likely bedifferent. Its effective impact ionization coefficient will be somewherebetween the coefficients of the individual bulk semiconductors. Second,the carrier effective mass will likely be different. The effective masssuperlattice will have no wells and barriers for carriers near the Γpoint in reciprocal space.

The unsuppressed band in a superlattice where the semiconductor pairband offset is entirely in the suppressed band will still have differenteffective masses for the two layers. Therefore, it forms an effectivemass superlattice where there are no wells for a carrier with littlekinetic energy (near the gamma point in reciprocal space) but wherewells gradually form as the carrier acquires kinetic energy. Dependingon the details of the materials forming the superlattice, the conductionband wells may be in the same layer as the valence band wells (Type Ialignment) or in opposite layers (Type II alignment). In the worst casefor an APD, the appearance of an effective mass superlattice can beginsuppressing carriers in the unsuppressed band. This effect can beameliorated by choosing a band offset such that the effective masssuperlattice disappears for carriers with some intermediate kineticenergy.

Next, again, FIG. 1 shows the graphs of the two bands overlaid on thesuperlattice multiplication region. The superlattice is set to beentirely in the Ev Valence band (thus, multiplying electrons but notholes). Therefore, the Ec conductance band is essentially a straightline sloping downwards. FIG. 1 shows an Epitaxial structure and energyband diagram of the proposed APD 100 under reverse bias. A photon (redwavy arrow) is absorbed in the p-InGaAs absorption layer, creating anelectron (black circle)—hole (white circle) pair. The hole diffuses tothe p-InP cap layer, and the electron diffuses to the multiplication.The electron is accelerated by the electric field until it accumulatesenough kinetic energy to impact-ionize a secondary electron-hole pair.Each electron then accelerates until it creates another electron-holepair. Because the impact-ionization-generated holes lose kinetic energyas they hop through the valence band superlattice, they cannotimpact-ionize. Thus, an enormous asymmetry in α _and β _is created.

Unusual as the one-sided band alignment may seem, there are severalpairs that are lattice-matched to either the III-V (where III and Vrespectively refer to columns IIIB and VB of the periodic table)semiconductor substrates InP or GaSb. A promising pair is InP andIn_(0.53)Al_(0.30)Ga_(0.17)As, in which both alloys are routinely grown.The alloy consists of 53% In, 30% Al, 17% of Ga for the group IIIcomponents, and entirely As for the group V component. FIG. 1 shows anAPD 100 built around the InP—InAlGaAs multiplication layer formultiplication of electrons by impact ionization without the concurrentmultiplication of holes. Electrons photogenerated with high quantumefficiency in the InGaAs absorption layer are injected into themultiplication layer. These electrons impact-ionize as they traversethrough the multiplication region. Secondary electrons can alsoimpact-ionize, but secondary holes cannot impact-ionize because theylose their kinetic energy as they hop from one quantum well to the next.The multiplication of electrons by impact ionization without theconcurrent multiplication of holes results in an exponential growth ofeach of the original photoelectron with low excess noise.

An arduous systematic search was conducted for matched pairs ofsemiconductors where the band offset is entirely in either the valenceband or the conduction band. A constraint for this semiconductor devicecan be that all of the semiconductor layers in a device withstate-of-the-art performance need to have a crystal structure with thesame type and the same dimensions as that of the substrate. Thisconstraint is known as lattice-matching. Without lattice-matching,crystal defects originating at the interface between layers degradeperformance. So our search for appropriate pairs needed to be performedseparately for each substrate of interest. For linear mode APDs in thenear infrared and shortwave infrared (approximately 0.8-3 μmwavelength), the most interesting substrates are InP and GaSb.

For these two substrates, InP and GaSb, we estimated the conduction bandenergy and the valence band energy for all lattice-matched alloys thatthe current epitaxial growth technologies would be able to grow. Forpairs with either equal conduction band energy or valence band energy,we tend away from those where with an alloy that consists of more thanfour elements since it would be difficult to control the stoichiometry,those with an alloy that has an indirect bandgap, and those with analloy known to phase separate.

The search yielded cases where an alloy A₁(x₁) that depends on aparameter x₁ characterizing the composition of the alloy over a range ofx₁ will have either the same conduction band energy or valence bandenergy as a second alloy A₂(x₂) over a range of x₂. We found that theequal conduction or valence band condition is met over a range of x₁that is synchronized with that of x₂. Within such a range, the endpointsof the range are most interesting because the band offset of the unequalband is maximized there.

Specific alloy pairs are given below. In the cases where the conductionor valence band energies can be the same over a range, we give pairsthat has the largest band offset for the unequal band for that range.

We have found semiconductor pairs that are useful for the linear modeAPD design both in terms of having the correct properties and in termsof being practical to do the epitaxial growth of the pair.

InP Substrate

The superlattice structure has lattice matched pairs of semiconductorsin the superlattice multiplication region, and the lattice matched pairsare also matched to an InP substrate, where the lattice matched pairsare selected from a group consisting of:

two semiconductors in the superlattice structure with a firstsemiconductor alloy of InP and a second semiconductor alloy ofIn_(0.53)Al_(0.30)Ga_(0.17)As with the superlattice being set in avalence band, where the hole is the first carrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of InP and a second semiconductor alloy ofIn_(0.19)Ga_(0.81)As_(0.69)Sb_(0.31) with the superlattice being set ina valence band, where the hole is the first carrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of InP and a second semiconductor alloy ofGaAs_(0.12)Sb_(0.61)P_(0.27) with the superlattice being set in avalence band, where the hole is the first carrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of In_(0.81)Ga_(0.19)As_(0.42)P_(0.58) and a secondsemiconductor alloy of In_(0.37)Ga_(0.63)As_(0.85)Sb_(0.15) with thesuperlattice being set in a valence band, where the hole is the firstcarrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of In_(0.94)Ga_(0.07)As_(0.12)P_(0.88) and a secondsemiconductor alloy of GaSb_(0.65)P_(0.35) with the superlattice set ina valence band, where the hole is the first carrier; or

two semiconductors in the superlattice structure with a firstsemiconductor alloy of In_(0.52)Al_(0.48)As and a second semiconductoralloy of In_(0.79)Ga_(0.21)As_(0.46)P_(0.54) with the superlattice beingset in a conduction band, where the electron is the first carrier.

GaSb Substrate

The superlattice structure has lattice matched pairs of semiconductorsin the superlattice multiplication region, and the lattice matched pairsare also matched to an GaSb substrate, where the lattice matched pairsare selected from a group consisting of

two semiconductors in the superlattice structure with a firstsemiconductor alloy of GaSb and a second semiconductor alloy ofIn_(0.57)Al_(0.43)As_(0.55)Sb_(0.45) with the superlattice being set inthe valence band, where the hole is the first carrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of Al_(0.14)Ga_(0.86)As_(0.01)Sb_(0.99) and a secondsemiconductor alloy of In_(0.09)Ga_(0.91)As_(0.08)Sb_(0.92) with thesuperlattice being set in the conduction band, where the electron is thefirst carrier;

two semiconductors in the superlattice structure with a firstsemiconductor alloy of Al_(0.44)Ga_(0.56)As_(0.04)Sb_(0.96) and a secondsemiconductor alloy of In_(0.28)Ga_(0.72)As_(0.26)Sb_(0.74) withsuperlattice being set in the conduction band, where the electron is thefirst carrier; or

two semiconductors in the superlattice structure with a firstsemiconductor alloy of In_(0.40)Al_(0.60)As_(0.42)Sb_(0.58) and a secondsemiconductor alloy of In_(0.79)Ga_(0.21)As_(0.72)Sb_(0.28) withsuperlattice being set in the conduction band, where the electron is thefirst carrier.

These compositions are approximate since the properties of the alloysare based on measurements made with varying uncertainties by manylaboratories.

Recognizing that these may be difficult to grow because of unintentionalintermixing of the two alloys at the interface, we consider methods thatwould relax the constraint on the structure without adversely affectingthe physical principles of the device.

The principle we use is that the offset does not have to be completelyeliminated in one of the bands. The offset only has to be small enoughthat for the design layer thicknesses there are no confined states lessthan kBT away from the continuum in that band, where kB is the Boltzmannconstant and T is the intended operating temperature. In the case of thevalence band, it is enough to consider only heavy hole states becauseelimination of the heavy hole confined states automatically eliminatesthe light hole confined states.

1. Including a Spacer Layer Between the Well and Barrier

For definiteness, we discussed the example alloy layers comprising ofIn_(0.52)Al_(0.48)As barriers and In_(0.79)Ga_(0.21)As_(0.46)P_(0.54)wells as an example. Rough surface morphology and low photoluminescencemay be a result of unintentional formation of AlP at the interfaces ofthe InGaAsP—InAlAs superlattice. A lattice-matched spacer layer ofInGaAs(P), which contains less P than the 54% in the well layer,inserted between the well and barrier layers will reduce AlP formationby having the Al-containing layer be adjacent to a P-containing layerwith lower P content than the well. The spacer will have little effecton the valence band if its valence band offset is small enough and it isthin enough that any confined hole. An In_(0.53)Ga_(0.47)As spacer thatis lattice-matched to InP is obviously the best spacer for preventingAlP formation, but the worst in terms of forming a valence band well.

Because the growth defines a direction to the superlattice, placing thespacer immediately after the growth of the InAlAs barrier layer is notequivalent to placing it immediately after the InGaAsP well layer.Starting with a baseline superlattice of 3.5 nm InGaAsP wells and 1.5 nmInAlAs barriers, we modeled three modified superlattice periods (lastlayer listed is grown first) (a) InGaAs(P)—InGaAsP—InAlAs, (b)InGaAsP—InGaAs(P)—InAlAs and (c) InGaAs(P)—InGaAsP—InGaAs(P)—InAlAs. Wekept the InAlAs barrier layer constant at the original thickness of 1.5nm and the sum of thickness of the other layers at the original wellthickness of 3.5 nm. The spacer is 1.5 nm thick for Cases (a) and (b),and each spacer is 0.5 nm thick for Case (c). The InGaAs(P) spacer layerwas modeled for 0%, 13%, 27% or 40% P content with the other componentsof the layer adjusted to maintain lattice-matching to InP. The modelingindicated that a spacer layer requires at least 13% P to prevent quantumconfinement of holes for Case (a), at least 27% P for Case (b) and atleast 40% P for Case (c). The effectiveness of a spacer layer with morethan 40% P to prevent AlP formation and the ability to grow 0.5 nmlayers are questionable, so Cases (a) and (b) would be promisingcandidates to reduce intermixing at the interface. Since only half ofthe interfaces will have a spacer for this example, it will work best ifthe intermixing occurs primarily in the InAlAs on InGaAsP or the InGaAsPon InAlAs interface.

2. All-Arsenide Superlattice Structure

Growth of a superlattice is more difficult the more atomic fractionsthat have to be changed in going from one layer to the next. Asuperlattice where both the well and the barrier are pure phosphides,pure arsenides or pure antimonides would be easier to grow than any ofthe ten listed above.

As an example of this, we consider the possibility of growing anall-arsenide lattice-matched superlattice with the relaxed property thatthe wells have localize quantum states in one of the bands but not inthe other. InGaAlAs wells and InAlAs barriers did not seem promisingbecause both the electron and the heavy hole were confined for layerthicknesses that are practical to grow. But a superlattice with acoupled well for each period appeared to be a way to decouple theelectron and the heavy hole confinement: the electric field pushes theelectron to, say, the right coupled well while it pushes the heavy holeto the left coupled well. With coupled wells, the right well parameterswill have a larger effect on the electron and the left well parameterswill have a greater effect on the heavy hole. A superlattice with aperiod consisting of 1.5 nm Q—1.5 nm InAlAs—3.0 nm Q—1.5 nm InAlAs whereQ=In_(0.53)Ga_(0.23)Al_(0.24)As and InAlAs=In_(0.52)Al_(0.48)As hasconfined electronic states with good well-to-well wavefunction overlapand no confined heavy hole states.

The superlattice where electrons or holes are suppressed consists ofalternating layers of alloy ‘A’ and alloy ‘B’ selected from the pairslisted above. All ‘A’ layers have the same thickness and all ‘B’ layershave the same thickness that may or may not be the same as the ‘A’ layerthickness.

As an alternative design to FIG. 1, FIGS. 5, 6 b, and 7 show a linearmode APD 500 with a matched superlattice design where electronmultiplication, but not the hole multiplication, is suppressed. Electroncannot accumulate enough kinetic energy to initiate multiplication. Onlyholes can initiate gain and cause potential exponential multiplication.Excess noise in APDs arises from simultaneous electron and holemultiplication. However, suppressing multiplication of either electronsand holes minimizes noise while providing gain. The matched superlatticeachieves multiplication of carrier at non-cryogenic temperatures such asroom temperature of 30-50 C, and even at temperatures above this range.The superlattice is designed to have at least one quantum state in eachof its well when it is biased to have an electric field that wouldsustain impact ionization in a thick layer of the well semiconductor orthe barrier semiconductor alone. In the case of a valence bandsuperlattice, there should be at least one quantum state for the lighthole and one quantum state for the heavy hole.

Next, to suppress the impact ionization for first carrier only, thedesign can chose pairs of semiconductors with their band offset mostlyin the band corresponding to the suppressed carrier. The band offset ofthe superlattice pair does not have to be entirely in the bandcorresponding to the suppressed carrier. There can be enough offset inthe opposite band as long as the offset is not sufficient to supportquantum states at the well and barrier thicknesses of the superlattice.This gives tolerance in the superlattice composition.

Graphically see FIG. 5, when the superlattice is set to be entirely inthe Ec Conductance band (thus, multiplying holes but not electrons), theEv Valance band is essentially a straight line sloping downwards.However, the coefficients of the materials can be matched to place thematerials mostly in one band, for example, the Conductance band butpotentially not entirely in that one band. Visually that would mean theother band, the Valance band would not be a solid straight line but agenerally straight line with occasional small little pulses which arenot deep enough to form a well. Thus, when the band offset of thesuperlattice pair is mostly set in the band corresponding to thesuppressed carrier, then the other carrier's band can have enough offsetas long as the offset is not sufficient to support quantum states at athickness of i) a well layer, ii) a barrier layer, or iii) either thewell layer or the barrier layer of the superlattice.

Additional Benefits of a Room-Temperature Linear Mode APD with a MatchedSuperlattice Structure

The proposed APD will be capable of operating as a low-noisephotocurrent amplifier with enough gain to enable capture of thephotocurrent waveform with single-photon sensitivity when its output iscoupled into commercially available electronic amplifiers. This devicewill achieve the ultimate detector sensitivity at room temperature bydetecting single photons at high quantum efficiency over a large band ofwavelengths.

The proposed device will not require Geiger mode operation to detectsingle photons. To illustrate the consequence of this, an array of SRI'slinear mode APDs can acquire a full three-dimensional point-cloud LIDARimage from a single shot of the laser. (See FIGS. 8 and 9) In contrast,an array of Geiger mode APDs requires a laser shot for each of the Nrange bins, increasing the acquisition time and laser energy N-fold.

Various Implementations to Suppress Electron- or Hole-Initiated ImpactIonization

A choice that needs to be made is whether we want to suppress electron-or hole-initiated impact ionization. For example, a conduction band onlysuperlattice and/or a valence band only superlattice. A major factorinfluencing this choice is the likelihood that the impact ionizationgenerates the suppressed carrier in a continuum state rather than aWannier-Stark state. To estimate this, we considered the kinematics ofimpact ionization in bulk InP. For electron-initiated impact ionization,we solved the conservation of energy and the two-dimensionalconservation of momentum equations under the effective massapproximation for these reactions:

e→e+e+H  (I)

e→e+e+h  (II)

where e is an electron, H is a heavy hole and h is a light hole, andwhere the initial electron has kinetic energy E. We calculated thethreshold for E where the reaction is allowed. Reaction I which producesa heavy hole has a threshold energy of 1.50 eV whereas reaction II whichproduces a light hole has a threshold energy of 1.99 eV. The thresholdenergy has a minimum value of the InP bandgap energy (1.34 eV), and ishigher by an amount needed to impart a final momentum to each of thefinal particles so energy and momentum are conserved. As the initiatingelectron energy increases above the 1.50 eV threshold, the rate at whichit can impact ionize increases as the density of final states increases.It is unlikely that it will reach the reaction II threshold. Withreaction I, the kinematics dictate that the heavy hole has an energy of0.10 eV below the valence band edge when the impact ionization occursnear the threshold energy. This places the heavy hole in the well (0.23eV deep) for the InP—InGaAlAs pair we use for electron-initiated impactionization.

For hole-initiated impact ionization, we consider these reactions:

h→h+e+H  (III)

h→h+e+h  (IV)

H→H+e+H  (V)

H→H+e+h  (VI)

The threshold is 1.52 eV for reaction III and 2.06 eV for reaction IV.We expect that the light hole will generate a heavy hole. The electronwill have an energy 0.03 eV above the conduction band edge when theimpact ionization occurs near threshold. For the heavy hole initiatedimpact ionization, the threshold is 2.63 eV for reaction V and 6.07 eVfor reaction VI. The electron will have an energy 0.04 eV above theconduction band edge when the impact ionization occurs near threshold.For both cases, the electron will near the bottom of the well (0.40 eVdeep) for the InAlAs—InGaAsP pair we use for hole-initiated impactionization.

The hole-initiated impact ionization structure is much better forpreventing unwanted carrier injection into the continuum states of thesuperlattice, though the electron-initiated structure may be adequate.

This analysis points out another factor that needs to be considered incomparing the two structures. When holes are initiating the impactionization, the threshold for generating a heavy hole is lower. Thevalence band in an APD will be filled with heavy holes. With a thresholdof 2.63 eV for heavy-hole-initiated impact ionization compared with 1.50eV for electron-initiated impact ionization, β for the hole-initiatedstructure will be about 1.8 times lower than the α for theelectron-initiated structure. So to obtain the same gain with bothstructures operating at the same electric field, the hole-initiatedstructure would need a 1.8 times thicker multiplication region and a 1.8times higher voltage.

The design calculates that a conduction band superlattice is better atsuppressing electron avalanche gain than a valence band superlattice atsuppressing hole avalanche gain.

Another advantage of the hole-initiated structure is that holes areinjected from the light absorption layer into the superlatticemultiplication region. Holes are injected from below, which allows theAPD to be made with zinc diffusion for select p-doping which is acommonly used to obtain low dark current and to eliminate surfacebreakdown in p-i-n photodiodes and APDs.

Neither structure is clearly better. The choice probably would depend onthe details of the intended application.

Applications

Various systems can employ an array of a plurality of linear modeavalanche photodiodes with a matched superlattice structure. Each linearmode avalanche photodiode is configured to sense light and outputelectrical current by being configured to, generate a gain equal to orgreater than 1000 times amplification while generating an excess noisefactor of less than 3 times a thermal noise present at or above anon-cryogenic temperature due to the gain from the amplification. Thelinear mode avalanche photodiode detects one or more photons in thelight by using a superlattice structure that is matched to suppressimpact ionization for a first carrier in the linear mode avalanchephotodiode while at least one of 1) increasing impact ionization, 2)substantially maintaining impact ionization, and 3) suppressing impactionization to a lesser degree for a second carrier. The first carrierhaving its impact ionization suppressed is either i) an electron or ii)a hole; and then, the second carrier is the electron or the hole. Apower supply is used to power the plurality of linear mode avalanchephotodiodes.

The systems can include i) a LIDAR system, ii) a night vision goggle orheadset system, iii) an optical communication system, iv) a spectroscopysystem, v) a Quantum key distribution system, vi) a system using highbreakdown voltage transistors, vii) a low-noise microwave generationsystem, and viii) a bio-medical system.

The linear mode avalanche photodiode, with the matched superlatticestructure is configured such that the amplification occurs in merely i)the conductance band or ii) the valance band, to generate a gain ofequal to or greater than 10,000 times amplification while generating anexcess noise of less than 10 percent of the thermal noise present at orabove a room temperature due to the gain from the amplification. Thelinear mode avalanche photodiode with the matched superlattice structurewith the gain of 10,000 times amplification is configured to be capableof detecting a single photon without a penalty of having a deadtimeafter the detection of the single photon where the linear mode avalanchephotodiode cannot detect another photon until after a set period oftime.

Example capabilities enabled by the linear mode APD described hereinclude:

FIG. 8 illustrates an embodiment of a diagram 800 of a linear mode APDarray with a matched superlattice structure with in-situ memory and aparallel column readout. Each pixel in array sees different x-ydirection, and acquires a z-direction. The array is designed for in-situstorage of each time bin, sequentially reading-out each time bin betweenlight pulses. The light pulses such as laser pulses may be captured in100 time bins in each 50 μm×50 μm pixel. Each time bin has a read outcircuit containing a linear mode APD with a matched superlatticestructure. FIG. 9 illustrates an embodiment of a diagram 900 of anexample read-out circuit schematic with linear mode APDs with a matchedsuperlattice structure. The readouts may occur at 100's frames/second.The readout circuit with 50 μm×50 μm pixel structure can store, forexample, 100 time bins.

The systems using the linear mode APD array with a matched superlatticestructure may include as follows.

Single shot light detection and ranging (LIDAR)—For LIDAR, a scene isilluminated by a short burst of light from a laser and reflections ofthis pulse is detected. The time between the emission of the laser lightand the detection of a reflection gives the distance to the reflector.The lack of a deadtime in the SRI linear mode APD allows for thecontinuous acquisition of the reflected light waveform with a singleAPD. Each APD in a linear mode APD array with proper optics can detectlight arriving from different directions. Each APD in the linear modeAPD array can acquire the full waveform for light reflected from objectslocated in a unique direction determined by the optics. Thus, a linearmode APD array can collect all reflections of a single laser shot over afield-of-view determined by the optics. The collected data represents athree-dimensional image of the scene where the arrival time gives thez-direction and each APD gives the x- and y-directions.

The waveform representing the data along the z-direction can be brokeninto time bins and the APD output current integrated over each time binstored as charge in a separate capacitor in the readout circuit. We havedetermined that a 50 μm×50 μm pixel can have 100 storage capacitors inaddition to the electronics needed to integrate the current and to readout the charge in the capacitors.

Because of the deadtime of Geiger mode APDs, two methods are used toacquire the all of the reflections. First, an array of parallelconnected Geiger mode APDs to form a superpixel can be used toameliorate the deadtime. The large size of the superpixel limits thearraying of superpixels to a linear array, so imaging in the directionorthogonal to the linear array is done either by illuminating the wholescene and rotating a narrow field-of-view APD array or by illuminating astripe of the scene collinear with a wide field-of-view APD array andmoving the stripe. This requires multiple laser shots to acquire thefull three dimensional image.

The second method uses a two-dimensional array of independent Geigermode APDs. Each APD collects reflected light from a different directionas in the linear mode APD array. The Geiger mode APDs are all gated openby biasing them above breakdown at a pre-determined delay after thelaser shot and for a pre-determined duration. The output of the arraygives the x-y image of the photon arrival during the time interval whenthe APDs are gated open. The output from a particular APD is the samewhether one or many photons arrived during that interval. By varying thegate delay, the full three dimensional image can be acquired withmultiple laser shots.

Advantages of acquiring the three-dimensional image in a single shot are(1) faster acquisition time and (2) lack of distortion and blurring dueto motion, both within the scene and between the LIDAR system and thescene. The faster acquisition makes possible three-dimensional video.The insensitivity to motion gives sharper images and relaxes therequirement for a very stable platform for the LIDAR system.

Imaging through obscurants—The principle is the same as for a singleshot LIDAR. The difference is that reflections due to obscurants, suchwater droplets in fog or smoke particles, occur randomly since theparticles are randomly distributed and are changing quickly with timesince they are undergoing Brownian motion, sedimentation or convection.Objects of interest are much more massive than obscurants so they movemore slowly and cannot change their velocity quickly. So by keeping onlyreflections that originate from the same point or that originate from aseries of points lying on a straight line over several consecutivethree-dimensional images, much of the reflections from obscurants can befiltered out.

Low light level imaging—In low light level imaging, it is oftendesirable to increase the photocurrent from a detector enough to beabove the noise floor of the electronic amplifiers. The linear mode APDcan be operated with modest gain, not enough to detect single photons,to do this. Alternative methods to do this are to increase the lightcollection aperture and to increase the integration time. Largerapertures result in larger optical components and bulkier, moreexpensive systems. Increased integration times require that the imagerbe on a stable mechanical support such as a tripod. Integration cannotbe used for a dynamic scene changing faster than the integration time.

An important case of low light level imaging is night vision. Withoutsunlight or an illuminator, the only illumination sources are moonlight,airglow and starlight, where airglow is emission in the visible butmostly in the near and short wave infrared originating in the ionospherefrom the recombination of ions created by sunlight earlier that day andby cosmic rays. Imaging without gain requires the light of a full moonto obtain a usable image at standard video rates with the lightcollection optics of a common handheld camera. Night vision goggles andimage intensifiers, which have gains of 10⁴ to more than 10⁶, can beused to image in overcast moonless nights where the illumination sourceis airglow and starlight scattered and attenuated by the clouds. Thegain provided by a linear mode APD without excess noise is similar tothat of night vision goggles and image intensifiers, so it is expectedto image in overcast moonless nights. A significant difference is themuch smaller size of the APD. Another significant difference is that theoutput of night vision googles is an image on a phosphorescent screenwhich cannot be read out for archiving or for image processing without aseparate camera, whereas the output of an imager made from an array oflinear mode APDs will be digitized.

Spectroscopy and detection of chemical and biologicalagents—Spectroscopy is operationally equivalent to low light levelimaging where the scene is the output of a dispersive element such as adiffraction grating. The gain of an array of linear mode APDs can beadjusted to give a modest gain to enough gain for signal photondetection, giving a large dynamic range. Moreover, the waveform seen bythe individual APDs in an array can be stored as with the single shotLIDAR application. Analogous to single shot LIDAR, this ability willenable the acquisition of the entire time-resolved spectrum triggered bya single event.

The ability to do spectroscopy at the single photon detection level canused to detect molecules at very low levels of concentrations.Time-resolved spectroscopy adds the ability to monitor molecularkinetics that can be provide information about the molecule'senvironment. The ability to detect trace amounts of specific chemicaland biological molecules is beneficial to the areas of security wherethese molecules can be illicit drugs, chemical or biological warfareagents or explosives, quality control where these molecules can be animpurity or unwanted byproduct of a manufacturing process, andbiomedical research where these molecules can be a new drug beingdeveloped.

Quantum key distribution—This requires the detection of single photonswith high quantum efficiency, preferably near room temperature. The roomtemperature, single photon detection technologies currently availableare photomultiplier tubes and Geiger mode APDs. Photomultiplier tubeshave very low quantum efficiencies, especially at the wavelengthstypically used for telecommunications. Geiger mode APDs effectively havelow quantum efficiency because of their deadtime. A linear mode APDwithout excess noise would be able to detect single photons with highquantum efficiency.

Optical communications—The dominant noise in most, if not all, opticalcommunications systems is thermal noise in the receiver electronics, sothe basic challenge in optical communications is to have an opticalsignal going into a receiver with sufficient power to exceed receivernoise equivalent input. Commonly used methods to ensure this are toincrease the power produced by the laser, to reduce the length of theoptical link to reduce losses and to put a repeater in the link. Alinear mode APD without excess noise at the receiver will increase theincoming optical signal in a less costly and less disruptive manner:lasers with high power output are expensive and the increased laserpower can introduce new difficulties such as nonlinear effects in theoptical fiber or the atmospheric transmission medium, link lengths areoften fixed by the application or by geography and cannot be reduced,and repeaters are very expensive, especially in regions like the oceanfloor where electrical power is not easily accessible.

High breakdown voltage transistors—The output power of a transistor islimited by its breakdown voltage. As discussed above, impact ionizationinitiated by a single carrier (α=0 or β=0) results in carrier densitiesthat are exponentially growing but that do not become arbitrarily large.Furthermore, the exponential growth of carrier density will besuppressed for a superlattice with α=β=0 achieved by designing localizedWannier-Stark states in both the conduction and valence bands. Carriertransport through the superlattice is by hopping. The hopping time canbe reduced to give carriers an effective velocity comparable to thesaturation velocity by increasing the wavefunction overlap betweenadjacent Wannier-Stark states.

The highest electric field in a bipolar transistor occurs in thecollector near the collector-base junction. Since the current flow in abipolar transistor is typically normal to the surface of thesemiconductor, it would be straightforward to replace the collector of abipolar transistor with a α=β=0 superlattice where the superlatticenormal coincides with the current flow.

Low-noise microwave generation—Impact ionization is a critical part ofthe operation of a class of microwave diodes which includes the Readdiode and the impact ionization avalanche transit time (IMPATT) diode.With impact ionization providing gain and the carrier transit timeproviding an appropriate phase shift, they are used a negativedifferential resistance element to generate microwaves. These generatorsare low cost and can produce high power, but the high phase noise intheir output have limited their application. The source of the noise isthe excess noise in the impact ionization mechanism. With thesuperlattice describe here as the impact ionization layer of the diode,excess noise will be suppressed and microwave generators built aroundthese diodes can be low cost, high power and low noise.

References in the specification to “an embodiment,” “an example”, etc.,indicate that the embodiment or example described may include aparticular feature, structure, or characteristic, but every embodimentmay not necessarily include the particular feature, structure, orcharacteristic. Such phrases can be not necessarily referring to thesame embodiment. Further, when a particular feature, structure, orcharacteristic is described in connection with an embodiment, it isbelieved to be within the knowledge of one skilled in the art to affectsuch feature, structure, or characteristic in connection with otherembodiments whether or not explicitly indicated.

While the foregoing design and embodiments thereof have been provided inconsiderable detail, it is not the intention of the applicant(s) for thedesign and embodiments provided herein to be limiting. Additionaladaptations and/or modifications are possible, and, in broader aspects,these adaptations and/or modifications are also encompassed.Accordingly, departures may be made from the foregoing design andembodiments without departing from the scope afforded by the followingclaims, which scope is only limited by the claims when appropriatelyconstrued.

1. An apparatus, comprising: a linear mode avalanche photodiode to sensephotons and output electrical current by being configured to, generate again equal to or greater than 1000 times amplification while generatingan excess noise factor of less than 3 times a thermal noise present ator above a non-cryogenic temperature due to the gain from theamplification, in order to detect one or more photons by using asuperlattice structure that is matched to suppress impact ionization fora first carrier in the linear mode avalanche photodiode while at leastone of 1) increasing impact ionization, 2) substantially maintainingimpact ionization, and 3) suppressing impact ionization to a lesserdegree for a second carrier, where the first carrier having its impactionization suppressed is either i) an electron or ii) a hole; and then,the second carrier is the electron or the hole.
 2. The apparatus ofclaim 1, where the linear mode avalanche photodiode with the matchedsuperlattice structure is configured in a superlattice multiplicationregion to suppress the impact ionization of the first carrier i) byimplementing a superlattice in a valence band when the first carrier isa hole and ii) by implementing a superlattice in a conduction band whenthe first carrier is an electron.
 3. The apparatus of claim 2, where aperiodic structure of layers of two or more materials forming thematched superlattice structure is designed to have at least one quantumstate in each of its wells when the superlattice is biased by a voltagecontroller to have an electric field that would sustain impactionization in a layer of a well semiconductor or a barrier semiconductoralone.
 4. The apparatus of claim 2, where the linear mode avalanchephotodiode with the matched superlattice structure such thatamplification occurs in merely i) the conductance band or ii) thevalance band is configured to generate a gain of equal to or greaterthan 10,000 times amplification while generating an excess noise of lessthan 3 times the thermal noise present due to the gain from theamplification, and where the linear mode avalanche photodiode with thematched superlattice structure with the gain of 10,000 timesamplification is capable of detecting a single photon without a penaltyof having a deadtime, which thus allows a detection of a subsequentphoton to occur substantially immediately after the detection of thesingle photon.
 5. The apparatus of claim 1, where the linear modeavalanche photodiode with the matched superlattice structure is matchedto suppress the impact ionization for only the first carrier in thesuperlattice multiplication region by using a periodic structure oflayers of two or more materials grown on a substrate that can be chosenfrom lattice matched pairs of semiconductors to form the superlatticestructure, with their band offsets set to i) at least mostly be in aband corresponding to the suppressed carrier and ii) up to entirely bein the band corresponding to the suppressed carrier where impactionization is to be suppressed.
 6. The apparatus of claim 5, where thesuperlattice structure has lattice matched pairs of semiconductors inthe superlattice multiplication region, and when the band offset of thesuperlattice pair is mostly set in the band corresponding to thesuppressed carrier, then the second carrier's band can have enoughoffset as long as the offset is not sufficient to support quantum statesat a thickness of i) a well layer, ii) a barrier layer, or iii) eitherthe well layer or the barrier layer of the superlattice, and where thelattice matched pairs of semiconductors forming the superlatticestructure comprises of alternating layers of a first semiconductormaterial and a second semiconductor material.
 7. The apparatus of claim5, where the superlattice structure has lattice matched pairs ofsemiconductors in the superlattice multiplication region, and thelattice matched pairs are also matched to an InP substrate, where thelattice matched pairs are selected from a group consisting of: a. twosemiconductors in the superlattice structure with a first semiconductoralloy of InP and a second semiconductor alloy ofIn_(0.53)Al_(0.30)Ga_(0.17)As with the superlattice being set in avalence band, where the hole is the first carrier; b. two semiconductorsin the superlattice structure with a first semiconductor alloy of InPand a second semiconductor alloy of In_(0.19)Ga_(0.81)As_(0.69)Sb_(0.31)with the superlattice being set in a valence band, where the hole is thefirst carrier; c. two semiconductors in the superlattice structure witha first semiconductor alloy of InP and a second semiconductor alloy ofGaAs_(0.12)Sb_(0.61)P_(0.27) with the superlattice being set in avalence band, where the hole is the first carrier; d. two semiconductorsin the superlattice structure with a first semiconductor alloy ofIn_(0.81)Ga_(0.19)As_(0.42)P_(0.58) and a second semiconductor alloy ofIn_(0.37)Ga_(0.63)As_(0.85)Sb_(0.15) with the superlattice being set ina valence band, where the hole is the first carrier; e. twosemiconductors in the superlattice structure with a first semiconductoralloy of In_(0.94)Ga_(0.07)As_(0.12)P_(0.88) and a second semiconductoralloy of GaSb_(0.65)P_(0.35) with the superlattice set in a valenceband, where the hole is the first carrier; or f. two semiconductors inthe superlattice structure with a first semiconductor alloy ofIn_(0.52)Al_(0.48)As and a second semiconductor alloy ofIn_(0.79)Ga_(0.21)As_(0.46)P_(0.54) with the superlattice being set in aconduction band, where the electron is the first carrier.
 8. Theapparatus of claim 5, where the superlattice structure has latticematched pairs of semiconductors in the superlattice multiplicationregion, and the lattice matched pairs are also matched to an GaSbsubstrate, where the lattice matched pairs are selected from a groupconsisting of: a. two semiconductors in the superlattice structure witha first semiconductor alloy of GaSb and a second semiconductor alloy ofIn_(0.57)Al_(0.43)As_(0.55)Sb_(0.45) with the superlattice being set inthe valence band, where the hole is the first carrier; b. twosemiconductors in the superlattice structure with a first semiconductoralloy of Al_(0.14)Ga_(0.86)As_(0.01)Sb_(0.99) and a second semiconductoralloy of In_(0.09)Ga_(0.91)As_(0.08)Sb_(0.92) with the superlatticebeing set in the conduction band, where the electron is the firstcarrier; c. two semiconductors in the superlattice structure with afirst semiconductor alloy of Al_(0.44)Ga_(0.56)As_(0.04)Sb_(0.96) and asecond semiconductor alloy of In_(0.28)Ga_(0.72)As_(0.26)Sb_(0.74) withsuperlattice being set in the conduction band, where the electron is thefirst carrier; or d. two semiconductors in the superlattice structurewith a first semiconductor alloy of In_(0.40)Al_(0.60)As_(0.42)Sb_(0.58)and a second semiconductor alloy of In_(0.79)Ga_(0.21)As_(0.72)Sb_(0.28)with superlattice being set in the conduction band, where the electronis the first carrier.
 9. The apparatus of claim 1, where a periodic setof materials for the matched superlattice structure is designed also tohave a spatial overlap of wavefunctions of the quantum states ofadjacent wells when under an applied bias such that the first carriertraverses the superlattice by hopping from a first well to a second welland substantially loses an amount of energy supplied by the applied biasto end up being less than a kinetic energy needed to impact ionize toform a new electron-hole pair for each hop from well to well.
 10. Amethod for sensing photons and outputting electrical current,comprising: configuring a linear mode avalanche photodiode to senselight and output electrical current by being to, generate a gain equalto or greater than 1000 times amplification while generating an excessnoise factor of less than three times of a thermal noise present at orabove a non-cryogenic temperature due to the gain from theamplification, in order to detect one or more photons by using asuperlattice structure that is matched to suppress impact ionization fora first carrier in the linear mode avalanche photodiode while at leastone of 1) increasing impact ionization, 2) substantially maintainingimpact ionization, and 3) suppressing impact ionization to a lesserdegree for a second carrier, where the first carrier having its impactionization suppressed is either i) an electron or ii) a hole; and then,the second carrier is the electron or the hole.
 11. The method of claim10, further comprising: configuring a superlattice multiplication regionin the linear mode avalanche photodiode with the matched superlatticestructure to suppress the impact ionization of the first carrier i) byimplementing a superlattice in a valence band when the first carrier isa hole and ii) by implementing a superlattice in a conduction band whenthe first carrier is an electron.
 12. The method of claim 11, furthercomprising: configuring a periodic structure of layers of two or morematerials forming the matched superlattice structure to have at leastone quantum state in each of its wells when the superlattice is biasedto have an electric field that would sustain impact ionization in alayer of a well semiconductor or a barrier semiconductor alone.
 13. Themethod of claim 11, further comprising: configuring the linear modeavalanche photodiode, with the matched superlattice structure such thatamplification occurs in merely i) the conductance band or ii) thevalance band, to generate a gain of equal to or greater than 10,000times amplification while generating an excess noise of less than threetimes the thermal noise present at or above a room temperature due tothe gain from the amplification, and configuring the linear modeavalanche photodiode with the matched superlattice structure with thegain of 10,000 times amplification to be capable of detecting a singlephoton without a penalty of having a deadtime, which thus allows adetection of a subsequent photon to occur substantially immediatelyafter the detection of the single photon.
 14. The method of claim 10,further comprising: configuring the linear mode avalanche photodiodewith the matched superlattice structure to suppress the impactionization for only the first carrier in the superlattice multiplicationregion by using a periodic structure of layers of two or more materialsgrown on a substrate that can be chosen from lattice matched pairs ofsemiconductors to form the superlattice structure, with their bandoffsets set to i) at least mostly be in the band and ii) up to entirelybe in the band corresponding to the suppressed carrier where impactionization is to be suppressed; and the second carrier's band will havesubstantially no band offset.
 15. The method of claim 14, where thesuperlattice structure has lattice matched pairs of semiconductors inthe superlattice multiplication region, and the lattice matched pairsare also matched to an InP substrate, where the lattice matched pairsare selected from a group consisting of: a. two semiconductors in thesuperlattice structure with a first semiconductor alloy of InP and asecond semiconductor alloy of In_(0.53)Al_(0.30)Ga_(0.17)As with thesuperlattice being set in a valence band, where the hole is the firstcarrier; b. two semiconductors in the superlattice structure with afirst semiconductor alloy of InP and a second semiconductor alloy ofIn_(0.19)Ga_(0.81)As_(0.69)Sb_(0.31) with the superlattice being set ina valence band, where the hole is the first carrier; c. twosemiconductors in the superlattice structure with a first semiconductoralloy of InP and a second semiconductor alloy ofGaAs_(0.12)Sb_(0.61)P_(0.27) with the superlattice being set in avalence band, where the hole is the first carrier; d. two semiconductorsin the superlattice structure with a first semiconductor alloy ofIn_(0.81)Ga_(0.19)As_(0.42)P_(0.58) and a second semiconductor alloy ofIn_(0.37)Ga_(0.63)As_(0.85)Sb_(0.15) with the superlattice being set ina valence band, where the hole is the first carrier; e. twosemiconductors in the superlattice structure with a first semiconductoralloy of In_(0.94)Ga_(0.07)As_(0.12)P_(0.88) and a second semiconductoralloy of GaSb_(0.65)P_(0.35) with the superlattice set in a valenceband, where the hole is the first carrier; or f. two semiconductors inthe superlattice structure with a first semiconductor alloy ofIn_(0.52)Al_(0.48)As and a second semiconductor alloy ofIn_(0.79)Ga_(0.21)As_(0.46)P_(0.54) with the superlattice being set in aconduction band, where the electron is the first carrier.
 16. The methodof claim 14, where the superlattice structure has lattice matched pairsof semiconductors in the superlattice multiplication region, and thelattice matched pairs are also matched to an GaSb substrate, where thelattice matched pairs are selected from a group consisting of: a. twosemiconductors in the superlattice structure with a first semiconductoralloy of GaSb and a second semiconductor alloy ofIn_(0.57)Al_(0.43)As_(0.55)Sb_(0.45) with the superlattice being set inthe valence band, where the hole is the first carrier; b. twosemiconductors in the superlattice structure with a first semiconductoralloy of Al_(0.14)Ga_(0.86)As_(0.01)Sb_(0.99) and a second semiconductoralloy of In_(0.09)Ga_(0.91)As_(0.08)Sb_(0.92) with the superlatticebeing set in the conduction band, where the electron is the firstcarrier; c. two semiconductors in the superlattice structure with afirst semiconductor alloy of Al_(0.44)Ga_(0.56)As_(0.04)Sb_(0.96) and asecond semiconductor alloy of In_(0.28)Ga_(0.72)As_(0.26)Sb_(0.74) withsuperlattice being set in the conduction band, where the electron is thefirst carrier; or d. two semiconductors in the superlattice structurewith a first semiconductor alloy of In_(0.40)Al_(0.60)As_(0.42)Sb_(0.58)and a second semiconductor alloy of In_(0.79)Ga_(0.21)As_(0.72)Sb_(0.28)with superlattice being set in the conduction band, where the electronis the first carrier.
 17. The method of claim 10, further comprising:configuring a periodic set of materials for the matched superlatticestructure to also have a spatial overlap of wavefunctions of the quantumstates of adjacent wells when under an applied bias such that the firstcarrier traverses the superlattice by hopping from a first well to asecond well and substantially loses an amount of energy supplied by theapplied bias to end up being less than a kinetic energy needed to impactionize to form a new electron-hole pair for each hop from well to well.18. A system, comprising: an array of a plurality of linear modeavalanche photodiodes, each photodiode with a matched superlatticestructure, where a first linear mode avalanche photodiode is configuredto sense light and output electrical current by being configured to,generate a gain equal to or greater than 1000 times amplification whilegenerating an excess noise of less than three times a thermal noisepresent at or above a non-cryogenic temperature due to the gain from theamplification, in order to detect one or more photons by using asuperlattice structure that is matched to suppress impact ionization fora first carrier in the linear mode avalanche photodiode while at leastone of 1) increasing impact ionization, 2) substantially maintainingimpact ionization, and 3) suppressing impact ionization to a lesserdegree for a second carrier, where the first carrier having its impactionization suppressed is either i) an electron or ii) a hole; and then,the second carrier is the electron or the hole; and a power supply topower the plurality of linear mode avalanche photodiodes.
 19. The systemof claim 18, where the system is selected from a group consisting of i)a LIDAR system, ii) a night vision goggle or headset system, iii) anoptical communication system, iv) a spectroscopy system, v) a Quantumkey distribution system, vi) a system using high breakdown voltagetransistors, vii) a low-noise microwave generation system, and viii) abio-medical system.
 20. The system of claim 19, where each of theplurality of linear mode avalanche photodiodes have a matchedsuperlattice structure that is matched to suppress the impact ionizationfor only the first carrier in the superlattice multiplication region byusing a periodic structure of layers of two or more materials grown on asubstrate that can be chosen from lattice matched pairs ofsemiconductors to form the superlattice structure, with their bandoffsets set to entirely be in the band corresponding to the suppressedcarrier where impact ionization is to be suppressed.